Present techniques for obtaining quantized video signals require multiple processing functions, each implemented by a combination of discrete and integrated circuits. The ability to extract a quantized video signal from degraded printing varies greatly between data lift systems, but is generally a function of the sophistication of the system.
A fairly complex system which has good thresholding characteristics with degraded printing is shown in block form in FIG. 1. The serial video signal from the single output part of an array of photosensitive elements is first amplified by a preamp in order to transmit it through an integrator. The signal is sampled at the end of the integration, and then converted to a 4 bit digital signal by the analog-digital converter. The AGC function serves to normalize the A/D output, always producing 16 gray levels for a range of video input levels.
The purpose of the converter is to drive the correlator function. The correlator is a digital filter that quantizes a 4 bit signal to a black or white level based upon the sum of the cells surrounding a cell. Approximately 20 MSI and SSI logic packages are required to implement the control function and timing signals that tie all the functions together.
The present invention has equal or greater thresholding characteristics and can be implemented in a single integrated circuit. This integrated circuit with several external components, replaces all the functional block shown in FIG. 1.
A parallel video processing system is described in U.S. Pat. No. 4,074,231, however in this patent a predetermined threshold value is used in processing the video. This does not allow for varying background shades or a variable threshold to allow for variances in the print contrast ratio.
Image processing systems similar to FIG. 1 are described in U.S. Pat. Nos. 3,947,817; 3,964,022; and 4,075,605.